1. Field of the Invention
The present invention relates to an injection molding machine having a tie-bar balance adjustment function.
2. Description of the Related Art
In closing a mold assembly for injection molding, mold surfaces (surfaces of stationary and movable molds) should preferably be brought into close contact with each other while maintaining a good parallelism between them, in order to improve the shape accuracy of molded articles. To attain this, a technique is proposed in which mold closing is performed while maintaining a good parallelism of movable and stationary platens of an injection molding machine. In producing clamping forces in the mold assembly by mold clamping, moreover, uniform surface pressures should preferably be applied to the mold surfaces to maintain satisfactory shape accuracy of molded articles and prevent generation of burrs. To this end, a technique is proposed in which a mold clamping unit is adjusted to equalize clamping forces produced by its four tie-bars.
Japanese Patent Application Laid-Open No. 11-77776 discloses a technique for maintaining parallelism of stationary and movable plates. According to this technique, one end of a screw shaft for actuating a toggle mechanism and nut gears for mold thickness adjustment, configured to be threadedly engaged with screw portions of tie-bars, are made to mesh with one another by bevel gears and connected by transmission means capable of being connected and disconnected by a clutch mechanism. In this arrangement, mold thickness adjustment is achieved by alternatively engaging or disengaging the clutch mechanism to rotate the nut gears independently of one another.
Japanese Patent Application Laid-Open No. 8-258102 discloses a technique for maintaining balance between clamping reaction forces that act on tie-bars. According to this technique, die-height adjusting nuts are arranged so that they can rotate independently of one another, and detection means is provided to detect tensions individually acting on the tie-bars or elongations of the tie-bars. In this arrangement, the die-height adjusting nuts are rotated so that the tensions on the tie-bars or the elongations of the tie-bars are equal during mold clamping.
If the mold clamping unit is adjusted so that the clamping forces produced individually by its four tie-bars during mold clamping are equal, however, the parallelism of movable and stationary platen surfaces of the movable and stationary platens may not always be satisfactory when the mold assembly is open. Movable and stationary molds are mounted on the sides of the movable and stationary platen surfaces, respectively.
The clamping forces produced by the tie-bars during mold clamping can be equalized by only adjusting the respective effective lengths of the tie-bars (i.e., lengths from tie-bar fixing portions on the side of a rear platen to those on the stationary-platen side) to be equal. Since the rear and stationary platens are connected by the tie-bars with the equal effective lengths, in this case, the rear and stationary platens are kept parallel to each other. In some cases, however, the parallelism of the movable and stationary platen surfaces of the movable and stationary platens that are connected by a power transmission system, such as toggle links, may not always be satisfactory. This is because the movable platen is tilted relative to the rear platen, since there are gaps for sliding at junctions between components of the toggle links and gravity acts on the toggle link components.
To adjust the movable and stationary platens to be parallel to each other in a state where the mold assembly is open, therefore, the respective effective lengths of the tie-bars are adjusted to correct the tilt of the movable platen. There is a problem, however, that the clamping forces produced by the tie-bars cannot be equal if mold clamping is performed in this adjusted state.
Thus, according to the conventional techniques, it is difficult to reconcile maintenance of the parallelism of mold surfaces in a state where the mold assembly is open with production of uniform clamping forces during mold clamping.
Normally, in an adjustment process during machine production, moreover, either the parallelism of mold surfaces or the uniformity of the clamping forces is preferentially adjusted, and readjustment after shipment would require a large-scale adjustment work.
Thus, regardless of whether the parallelism of mold surfaces or the uniformity of the clamping forces affects the quality of molded article, molding is generally performed in a state that has been adjusted at the time of factory shipment, without readjusting the mold clamping unit.